Process for the production of coloured coatings



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QERALD P.. Ew/5 I Avent United States Patent Office 3,530,013 Patented Sept. 22, 1970 3,530,013 PROCESS FOR THE PRODUCTION OF COLOURED COATBNGS Robert W. Smyth, Oakville, Ontario, and Gerald P. Lewis, Streetsville, Ontario, Canada, assignors to Cominco Ltd.,

Montreal, Quebec, Canada, a corporation of Canada Filed Aug. 24, 1966, Ser. No. 574,684 Int. Cl. C23f 7/02 U.S. Cl. 14S-6.3 15 Claims ABSTRACT OF THE DISCLOSURE A process for the production of coloured surfaces on zinc coatings by the provision of oxide films having light interference effects. A` molten alloy of zinc with a minor amount of an oxygen-avid element such as titanium, manganese or vanadium is oxidized Iby exposure to a freeoxygen containing gas under controlled time and temperature conditions for the provision of a surface film of an oxide of the oxygen-avid addition element having light interference colour characteristics.

This invention relates to a process for the production of coloured coatings and is particularly directed to a process for the production of coloured zinc coatings and to zinc alloy coating compositions therefor.

The production of coloured metal surfaces by oxidation of a metal is known as, for example, in the tempering of steels to increase toughness, the degree of tempering being characterized by specific surface colours. The tempering process, and attendant colour which is a measure of the degree of tempering, can be arrested as desired by quenching.

The formation on ferrous bodies of an oxide film having a thickness characterized by a colour ranging from light yellow to purple and gray for subsequent reconversion to the parent metal by reduction is taught in Sendzimir U.S. Pat. 2,197,622.

It is also known to colour metal surfaces, particularly iron or nickel, by forming a layer of less than 2 microns thickness on the surface of the metal by electrodepositing lead oxide thereon, as is taught in British Pat. No. 1,010,065.

It is not known, however, how to provide coloured coatings on zinc surfaces in a predictable and facile manner by the formation of an oxide film thereon, having light interference properties. Such coloured coatings are not formed under normal galvanizing conditions.

We have found that the use of a minor amount of certain oxygen-avid addition agents in a zinc bath results surprisingly in the production on metallic articles of adherent coatings having attractive and predictable surface colours and textures.

We have thus discovered a novel process for the production of coloured zinc coatings on articles and surfaces comprising, in general', the steps of forming a bath of molten zinc having present in the bath an oxygen-avid element selected from the group consisting of titanium, manganese, Vanadium, columbium, zirconium, thorium and mischmetal, applying the alloy composition to a surface of the article such as by dipping said article in said bath, and contacting the resulting molten coating with a free-oxygen containing gaseous atmosphere to permit reaction of the molten alloy composition with oxygen for the provision of a thin oxide film having desirable light interference colour characteristics and effects.

We have also discovered that the presence of cadmium, arsenic, copper, lead or chromium in a zinc bath at elevated temperatures of at least about 625 C., results in the production of coloured coatings when the alloys thus formed are applied to surfaces and oxidized in the manner described above.

It is a principal object of the present invention, therefore, to provide a process for the production of coloured zinc alloy coatings.

It is another object of the present invention to provide controlled formation of consistent colours on coatings on the surfaces of workpieces in a manner adaptable for use with conventional galvanizing techniques.

And another object of the invention is the provision of a tightly adherent coloured galvanized coating having the normal corrosion resistant properties of zinc.

Additional objects and the manner in which they can be attained -will become readily apparent to one skilled in the art from the following detailed description of the invention, reference being had to the following drawings, in which:

FIG. 1 is a graph illustrating the effect of bath temperature and composition on yellow colour formation for an Zn-Mn alloy;

FIG. 2 is a graph illustrating the effect of bath temperature and cooling rate on variable colour formation for a Zn-Mn alloy;

FIG. 3 is a graph illustrating the effect of bath temperature and cooling rate on variable colour formation for a Zn-Ti alloy; and

FIG. 4 is a graph illustrating the effect of bath temperature and cooling rate on variable colour formation for a Zn-V alloy.

In accordance with the process of the present invention, coloured zinc alloy coatings can be produced on surfaces of various metals such as iron, steel, copper, nickel, zinc and other metals, on surfaces of zinc-coated articles and on surfaces of nonmetallic materials such as graphite by applying to said surfaces a coating of zinc having alloyed therewith an oxygen-avid addition agent such as manganese, titanium and vanadium in amount sufficient to form, upon reaction of the surface of said coating with oxygen, an oxide film on said coating having light interference colours. Any material which is amenable to receiving a coating of the zinc alloy, under suitable temperature conditions and other operating conditions discussed hereinbelow, can be colour coated according to the process of the invention.

The colours formed on the surfaces of zinc coatings containing alloying elements are due to light interference effects produced by transparent oxide films formed on the metal surfaces and are caused by the destructive interference of light waves reflected from the front and back surfaces of the film. When light Waves are reflected from a thin transparent film, those reflected from the back surface are retarded with respect to those from the front surface by an amount 2er, where n is the index of refraction of the film and t is the thickness. 'I'his assumes normal incidence and ignores any difference in phase change due to reflection at the two interfaces. When the retardation is equal to an odd number of half-wavelengths, destructive interference will occur.

Thus the film thickness requirement for destructive interference is given by where z' is the film thickness, N is a small whole integer and C reprseents the thickness equivalent to the differences in phase change at the two film surfaces.

As the thickness of a thin film is increased the first colour effects caused by the formation of an interference band occur when where a, is the wavelength of violet light. The surface will then have the complementary yellow or gold colour. The effects of further increases in film thickness are illustrated in Table I below wherein a number of test runs conducted according to the process of the invention on various alloys of manganese, titanium or vanadium with zinc established the following colour sequence.

TABLE I First order: Third order:

Yellow (gold) Yellow Red Red Blue Green Silver Fourth order: Second order: Yellow (narrow range) Yellow Red Red Green Blue Fifth order: Green Red It is believed that with an increasing thickness of the film an interference band moves across the spectrum and the resultant colour changes from gold to red to blue. When the thickness increases to the point that the first interference band passes out of the visible spectrum, there is a gap in the colour sequence and the film appears silvery. This is followd by a series of second order colours as a second interference band moves across the spectrum. The bands giving rise to higher order colours are increasingly closer together so that the colour sequences may differ from those of the first order. Thus, at the end of the second order colours the second and third interference bands are affecting the red and violet ends of the spectrum respectively, with the result that the film appears green for the first time as indicated in Table I. As the film thickness increases further, several interference bands occur in the visible spectrum at the same time and the colouring effects beyond the fourth order decrease and disappear.

The characteristics of the interference bands are dependent on the index of refraction and absorbing power of the film, as well as on the reflectivity of the surfaces. Hence, differing colour `shades and intensities and to some extent, different colour sequences result from interfereence effects on films of different materials.

The above sequence of colour occurrences is reproducible and it will be seen, with reference to FIGS. 2-4, that the coating colour selectively attained can be achieved in part by regulating the length of effffective time of reaction of oxygen with the zinc alloy for the growth of an oxide film.

The Zinc-alloy coating can be applied to the metal surfaces by spraying said alloy in molten form onto said surfaces with or without subsequent heat treatment, or by immersing or dipping said surfaces into a bath of the molten alloy. The thus-coated surfaces are contacted with a free-oxygen containing gas such as air and preferably cooled in said atmospheer for the formation of a thin, oxide film on the coating producing the desired colour; the eventual colour achieved being controlled by the thickness of the oxide film which is dependent upon the alloy composition, alloy temperature, and the period of time the coating is permitted to react with oxygen, i.e., the rate of cooling relative to the initial coating temperature.

Although the description will now principally proceed with particular reference to binary alloys of zinc with manganese, titanium, or vanadium, it will be understood that the invention is intended to include binary, ternary, quaternary and the like alloys of the said addition agents with zinc as well as the above-mentioned addition agents of the group consisting of columbium, zirconium, thorium and mischmetal, and the group consisting of cadmium, arsenic, copper, lead and chromium.

With reference now to FIG. 1, the effect of bath or initial coating temperature and composition of zinc-manganese baths is shown relative to time of contact with the oxygen in air for the occurrence of first, second and third orders of the colour yellow on the melt surface, the manganese content of the Zinc bath being controlled at levels of 0.04%, 0.07%, 0.11% and 0.33% by weight. It will be evident from the graph that the occurrence of first order yellow at manganese concentrations of 0.11% and 0.33% was almost instantaneous at all temperatures above about 419 C., ie., the melting point of the alloy. At manganese concentrations of 0.07% and below, however, considerable time was necessary for the occurrence of first order yellow even at bath temperatures up to about 480 C. and 500 C.; the occurrence of first order yellow for a 0.04% manganese content occurring only at a temperature above 520 C. after 50 seconds of exposure to oxygen. Thus, a range of manganese contents above 0.07% by weight in the zinc is desirable at process temperatures above about 500 C., the practical lower limit being about 0.1% concentration above which increases in the manganese content do not appreciably increase the rate of colour formation. The upper operable limit is determined by the solubility of the manganese in the zinc at the operating temperature. The upper practical limit is the eutectic composition.

Comparable tests conducted on zinc-vanadium alloy baths established the occurrence of first order yellow at a bath temperature of 500 C. at 8 seconds for a 0.018% by weight vanadium content, 15 seconds for a 0.011% by weight vanadium content, and 19 seconds for a 0.009% by weight vanadium content. The first order yellow appeared at 3 seconds at vanadium concentrations in the zinc of 0.076% and 0.46% by weight at an alloy bath temperature of 500 C. The colour occurrence was therefore relatively consistent at vanadium contents by weight in the zinc at and above about 0.075%, the practical lower limit being about 0.1 by weight. The upper operable limit is determined by the solubility of vanadium in zinc at the operating temperature and the upper practical limit is determined by the eutectic composition.

Tests conducted on zinc-titanium alloy baths established the occurrence of first order yellow at a temper- `ature of 500 C. at 7 seconds for titanium contents in the zinc of 0.09% and 0.16% by weight. Colour occurrence was relatively consistent at titanium concentrations down to 0.008% by weight and at concentrations below 0.008% by weight the rate of colouration decreased rapidly. The lower practical limit is 0.1% by weight titanium in Zinc `and the upper practical limit is the eutectic composition. The upper operable limit is determined by the solubility of titanium in zinc at the operating temperature.

The foregoing alloy composition ranges and limits of manganese, vanadium and titanium with Zinc were established by colour occurrence on the respective alloy bath surfaces. Colour occurrences were noted for each alloy composition at concentrations as low as 0.0001% by weight. Colour occurrences on dipped articles were noted at manganese, vanadium and titanium concentrations by weight in zinc of 0.02%, 0.001% and 0.001% respectively at alloy bath temperatures of 600 C., 650C. and 650 C. respectively, as will be evident from the following example. It will be understood that although the preferred and practical composition ranges and limits were determined by colour formation on bath surfaces, the parameters apply equally to dipped articles, only the time of colour formation being changed because of the effect of cooling of the article by exposure to air. Prolonged retention of dipped articles at elevated temperatures in a free-oxygen containing environment such as air can be controlled to approximate bath temperature conditions and hence rate and extent of colour formation are similar.

The preferred lower limit for manganese, vanadium and titanium of 0.1% by weight in the zinc permits cornpensation for loss of the alloying element in the bath. The upper practical limit for alloys of manganese, vanadium and titanium with zinc determined by the respective eutectic composition avoids precipitation of the addition agents from the solutions upon temperature variation of the bath which can be altered to provide an expedient technique for control of coating colour. The eutertic composition for the above alloys of manganese and titanium with zinc can be obtained from the text by Hansen entitled The Constitution of Binary Alloys. The eutectic com position for zinc-vanadium alloy was obtained from Trausactions of the Metallurgical Society of AIME, vol. 227, page 485, 1963. The preferred range for the alloy compositions of manganese, vanadium and titanium with zinc is therefore from about 0.1% by weight to about the eutectic composition of the respective alloying element in Zinc, that is, from about 0.1% to about 0.45% for manganese and titanium, and from about 0.1% to about 0.15% for vanadium.

The following example describes concentration effects at lower composition levels on colour occurrences on dipped samples coated with compositions of manganese, titanium and vanadium alloyed with zinc. A number of tested were conducted in which specimens of galvanized sheet steel of three thicknesses, $0-, 24- and 16-gauge,

were dipped in baths of zinc-manganese alloy within the temperature range of 500 to 600 C. and Zinc-titanium and zinc-vanadium alloys within the temperature range of 500 to 650. All specimens were allowed to reach the bath temperature before withdrawal for exposure to arnbient air conditions for coating solidication.

Initial tests were conducted wherein the bath temperatures were maintained constant and the alloying agents were diluted until no colouration was observed on dipped samples. The bath temperatures were then raised in steps, maintaining the alloy composition, until colour was again produced on the dipped samples. This procedure of diluting the alloying agents to rst eliminate colour and then raising the bath temperature to produce colour at the same alloy composition Was continued until no colouration on dipped samples occurred at the established upper temperature limits. Assay samples were taken for each dilution step and, together with colour observations, were tabulated as shown in Table Il. The rst three zinc-manganese bath compositions represent calculated values and the remaining compositions in Table II represent Wet analysis results.

TABLE IL DIIPIN (1r-RESULTS Coating Bath Freezing Temp. Material Time Bath Analysis (by weight) C.) (gauge) (ser.) Final Coating Colour Zn-Mn Alloys:

Ztl-0.045% Mn 500 16 65 1st Red-light.

500 24 32 1st Yellow. 500 30 26 1st Yellow-very light. Zn0.04% M11 500 16 72 lst Yell0w-very light.

500 24 40 D0. 500 30 29 No colour. Zn0.03% Mn 500 16 65 D0. 500 24 42 D0. 530 16 91 1st Yellow-very light. 530 24 51 No colour. 550 16 90 1st Yellow-very light. 544 24 54 No colour. 544 30 37 D0. 570 16 100 1st Yellow. 570 24 50 1st Yellow-very light. 570 30 32 No colour. Ztl-0.02% Mn 590 16 98 1st Yellow-Very light.

590 24 45 No colour. 610 16 111 1st Yellowvery light. 610 24 48 No colour'. Zn4Ti Alloys:

Zn-0.074% Ti 550 16 72 1st Blue.

550 24 35 Do. 550 30 26 lst Red-light. Zn-0.062% Tl 550 16 68 1st Silver Blue.

568 24 35 1st Blue. 562 30 25 Do. zii-0.03% Ti 550 16 70 1st Silver Blue.

555 24 34 1st Blue. 550 30 :26 1st Red. Z1i-0.0l7% Tl 555 16 75 2d Yellow.

555 24 `33 1st Blue. 550 30 28 1st Red. Zn-0.008% Tl 550 16 71 1st Blue.

545 24 38 1st Red. 540 30 29 1st Yellow. Zn-0.0035% Ti 555 16 77 No colour.

545 24 35 D0. 545 30 224 Do. 570 16 B2 1st Yellow-light. 570 24 V32 No colour. 575 30 28 Do. 585 16 85 1st Yellow-very light. 615 16 :B5 D0. 615 24 `56 No colour. 652 16 '98 1st Yellow-very light. 650 24 .37 N o colour. Zn-0.0015% Ti 602 16 70 D0. 620 16 85 D0. 645 16 90 1st Yellow-very light. 666 24 40 No colour. Zn-V Alloys:

Zn-0.014% V 550 16 B0 1st Blue.

560 24 L32 1st Red-very light. 560 30 23 1st Yellow-light. Zn-0.0l2% V 550 16 75 D0. 555 24 45 Do. 555 30 27 D0. Zn-0.0065% V 550 16 B0 1st Blue.

555 24 40 1st Red-very light. 555 30 128 1st Yellow-light. Zn-0.004% V 555 16 L85 1st Yellow-light.

555 24 41 1st Yellow-very light. 555 30 29 No colour. Zyl-0.002095 V 555 16 82 Do. 555 24 40 D0. 600 16 90 Do. 640 16 92 Do. 650 16 '94 1st Yellow-very light. 655 24 55 N o colour.

The following example illustrates the effect of regulating the length of effective time of reaction of oxygen with the zinc alloy. A series of 16-, 24- and 30-gauge pregalvanized panels and 1/2 diameter rods were dipped itno melts of zinc containing 0.1% manganese, zinc containing 0.15% titanium and zinc containing 0.15% vanadium. The panel immersion time and bath temperature were varied and the freezing time of the coating and final colour were noted. At each temperature level, the melt surface was skimmed and colour formation was also timed.

Results plotted on the graph in FIGS. 2, 3 and 4 show the effect of cooling rate and bath temperature on the formation of colours on the surface of dipped articles. With reference to the graphs, thc areas bounded by fine solid lines are the colours observed on the surface of the melt, representing a zero cooling rate, and the areas bounded by heavy solid lines are the final colours that are formed on the surface of dipped articles, both upon exposure to air. The areas defined by lboth groups of solid lines indicate the actual colours observed. The broken lines represent the coating freezing times for 30, 24- and 16- gauge sheet specimens, air cooled at room temperature of 20 C., and the heavy solid-lined areas intersected indicate the nal colours that form on the specimen surfaces when dipped at particular temperatures.

The observations tabulated in Table III may be made from the graphs of FIGS. 2-4; in all cases immersion times were sufficient for the specimens to attain bath temperature and the coatings were subsequently air cooled at room temperature (20 C.).

The foregoing examples relates to colours achieved where the dipped article was allowed to air cool from the bath temperature. By quenching the article with cold air blasts, a colour appearing earlier in the colour sequence is formed. Immersion times insuiiicient to attain the bath temperature also form colours appearing earlier in the sequence. By decreasing the cooling rate or increasing bath temperature a colour appearing later in the colour sequence can be formed, as for example, where an article is dipped, held at both temperature (zero cooling rate) for a prolonged time, and then rapidly cooled -by quenching when the desired colour had formed.

The graphs indicate that the effect of cooling rate on colour formation is more pronounced on the zinc-manganese (Zn-0.1% Mn) alloy than on either the zinctitanium or zinc-vanadium (Zn-0.15% V) alloys.

The foregoing Sendzimir patent, illustrative of the disclosures of the prior art, teaches that the presence of from 0.001% to 0.35% aluminum in zinc is necessary for effective bath control in the continuous galvanizing of metal surfaces with zinc. In general, small additions of aluminum in amount of about 0.003% normally are made to zinc baths for control of surface dross in the galvanizing of strllctural shapes and fabricated articles. We have found that the presence of about 0.002% or more by weight aluminum in the zinc alloy bath precludes formation of a desirable oxide lm having light interference effects. The presence of as little as 0.0005% by weight aluminum, while not sufficient to prevent colouration, does -decrease the rate of colour formation sufficiently to impede the operation of the process of the present invention.

Although it will be understood the invention is free from hypothetical considerations, it is believed that the presence of the aluminum in amounts of 0.0005% and more results in the aluminum preferentially oxidizing to form a protective film of A1203 which prevents the formation of, for example, T102, V205, MnO or ZnO oxide films. In that the A1203 layer is extremely thin, n0 light interference colours are obtained.

In the following tests, conducted to establish deleterious 10 alumlnum concentrations, the aluminum content of zinc alloy baths was increased from 0 to 0.005% by weight.

Melts of commercial Special High Grade zinc (99.99v+%) with titanium (Zn-0.15 Ti), manganese (Zu-0.15% Mn) and vanadium (Zn-0.15% V) were held at a constant temperature, and aluminum in the form of a zlnc aluminum alloy (Zn-1.0% Al) was added to each melt to increase the concentration of aluminum by lllcrements by weight of 0.0005 The rate of color formatlon on the surface of the melt and the colour formed on the dipped panel were noted after each addition of alumlnum. The results are listed in Table IV.

TABLE IV l Rate of colour formation Colour of di ed Conlposltions on melt surface Panel pp Zn-0.l5% Mn at 500 C.: oller yelllow, 1 see.

. or er yel ow 3 sec Il 015% MH 3rd Order yellow 14 See" Red (2d 01(101).

4th order yellow, 30 sec t yelllow,4l sec G 1d ye ow, Sec 0 lst ZnA'llZj M 0000570 3rd yellow, 20 sec yellow).

""""""""" 4thlyellow, not

o isel'vcd. Zwalm, M11-0.00152, ust yellow, 3 se@ l A1 lst Silver, 100 sec.. G0 d' Yellow, 3 see D Some red at 100 sec. o' Yellow, 6 see D No change to 100 see. o' 40 ZIX).15%, Mn-O. 003% Lltglloyfllow, no change Light gold.

. O SSC. Ztl-0.15%, Mn-O. 0035% Very light yellow, no No colour.

Al. challge to 100 scc. Z1).15%, Mn-0.004% No colour Clear and zll-0. 15% Tl er 550 o.: b1 ight' 1st order yellow, 2 sec... 40 Z1l-0.l5% Ti 2d order yellow, 42 sec. Bloze. 3rd order yellow, 130 sec.. 01 @l le Zn-0.15%, T10.0005% 1st order yellow, 6 scc..- B

. 1st order red, 100 seem-. mme Zn-0.15%, 'Fl-0.001% Yelliw, sec.; some Gold (lst re at sec. yellow). 2x1-0.015%, T1-0.0015% Light yellow, no change Light gold. A1. after 100 sec.

Zn-0.15%, Tl-0.002% Very light yellow, no Do.

Al. change after 100 scc. Zrtlllb, Tl-0.0025% No colour after 100 sec. Do. Zn-0.15%, Ti-0.003% do Light gold, some c ear areas. Ztl-0.15%, T1-0.0035% do Very light gold 4 111 SOIIIG areas. Zn-0.15%, T1-0.004% do Very light gold Al. tint in some areas. Z11l1).15%, Ti-0.0045% do Clear. Zn-0.15%, Tl-0.005% ..do Clear and Al. bright.

ZI1-0.15% V at 500 C.:

t lst order yellow 2.5 sec. Red 1st Zu 0'15% V- l2d order yellowf() sec. -i order). Zn-O.15%, V-0.0005% {lst yellow, 3 sec Gold (1st Al. 1st sllver, 100 sec yellow). Zn-0.15%, V-0.001% Al Yellow, 4 sec.; some Gold, some red at 100 sec. clear areas.

(colours very pale). Ztl-0.15%, V-0.00l5% Light yellow, 11o change Clear.

to 100 see. Ztl-0.15%, V0.002% Al- Very light yellow no Do.

change to 100 sec. Zn0.l5%, V-0.0025% No colour, clear alld Clear and Al. bright. bright.

Table V below lists the results of coating specimen -panels according to the process of the invention with zinc containing alloying elements from the group columbium, zirconium, thorium and mischmetal and the group cadmium, arsenic, copper, lead and chromium.

TABLE V Bath Temperature, C. Colour Sequence Bath Analysis (by Weight) Gold Ztl-0.1% Cb 50G-650 {111rple -.}Light shades.

Gold.` 550-650 Purple Ztl-2.0% Th Zn-0.5% mschmetal Zn5.010% Cd Zn-1.0% Cr Alloying compositions of zinc with titanium, manganese, vanadium, columbium, zirconium, thorium or mischmetal provide coloured coatings on steel and pregalvanized materials at temperatures within the range of from about 419 C., i.e., the melting point of the alloy composition, to about 600 C. and above. Alloying compositions of zinc with cadmium, arsenic, copper, lead or chromium provide coloured coatings on said substrates at temperatures of at least about 625 C.

It has also been discovered that the presence of titanium, manganese or vanadium in amounts of about 1.0% by weight in molten tin at an alloy bath temperature at about 500 C. permits production of coloured coatings on iron and steel surfaces by the process of the present invention. Although not as brilliant as zinc alloy colours, a complete range of colours has been produced with tin alloys at a bath temperature of 500 C.

The present invention provides a number of advantages. Colours produced by the process and compositions of the invention are reproducible and can readily be controlled by varying one or more of alloy bath composition and temperature and the rate of cooling of the molten surface in a free-oxygen containing atmosphere, i.e., varying the length of the time the alloy remains in its molten and relatively reactive state. Variegated colours, patterns and textures overlying zinc coatings can be produced providing, in combination, aesthetic effects andcorrosion resistant properties. The coatings can be applied to substrates such as steel or zinc-coated materials in the form of sheet, wire, and formed articles such as expanded mesh, pipe and structural members.

It will be understood, of course, that improvements can be made in the process and apparatus of the present invention described hereinabove vwithout departing from the scope and purview of the appended claims.

What we claim as new and desire to protect by Letters Patent of the United States is:

1. A vprocess for the production of a coloured coating on an article comprising the steps of forming thereon a molten coating of an alloy of zinc and an oxygen-avid alloying element selected from the group consisting of titanium, manganese, vanadium, columbium, zirconium, thorium, mischmetal, cadmium, arsenic, copper, lead and chromium, said alloy having less than 0.002% by weight aluminum, said oxygen-avid alloying element being present in amount effective to produce an oxide -lm having discernible light interference colour eitects, contacting said molten coating with a free-oxygen containing gas for reaction therewith to provide thereon an oxide ilm having said colour eitects, and solidifying said molten coating provided with said oxide lm.

2. A process for the production of a coloured coating on a surface comprising the steps of applying thereto an alloy of zinc and an oxygen-avid alloying element selected from the group consisting of titanium, manganese, vanadium, columbium, zirconium, thorium, mischmetal, cadmium, arsenic, copper, lead, and chromium, said alloy having less than 0.002% by weight aluminum and said oxygen-avid element being present in amount effective to produce an oxide lm having discernible light interference colour effects to form a molten adherent coating thereon, forming an oxide lm on said coating having said colour elects by reacting said coating with an oxygen-containing atmosphere, and solidifying said molten coating provided with said oxide film.

3. A process for the production of a coloured coating on a surface comprising the steps of forming a substantially aluminum-free bath of zinc having an oxygen-avid alloying element therein selected from the group consisting of titanium, manganese, vanadium, columbium, zirconium, thorium, mischmetal, cadmium, arsenic, copper, lead and chromium, and present in amount effective to produce an oxide ilrn having discernible light interference colour effects, immersing the surface in said bath to form an adherent molten coating thereon, contacting said coating with a free-oxygen containing gas to provide an oxide lm having said colour effects, and solidifying said molten coating provided with said oxide lm.

4. In a process as claimed in claim 3, said oxygen-avid alloying element being selected from the group consisting of titanium, manganese, vanadium, columbium, zirconium, thorium and mischmetal and said molten coating having less than 0.002% by weight aluminum.

5. In a process as claimed in claim 4, cooling the molten said coating in air for the provision of a zincalloy oxide film.

6. In a 4process as claimed in claim 3, said oxygen-avid element being selected from the group consisting of cadmium, arsenic, copper, lead and chromium, said molten coating having less than 0.002% by weight aluminum.

7. In a process as claimed in claim 3, said oxygen-avid alloying element consisting of at least about 0.02% by Weight manganese and said bath having less than 0.002% by Weight aluminum.

8. In a process as claimed in claim 3, said oxygen-avid alloying element consisting of at least about 0.07% by weight manganese and said bath having less than 0.002% by weight aluminum.

9. In a process as claimed in claim 3, said oxygen-avid alloying element consisting of at least about 0.001% by Weight titanium and said bath having less than 0.002% by weight aluminum.

10. In a process as claimed in claim 3, said oxygen-avid alloying element consisting of at least about 0.008% by weight titanium and said bath having less than 0.002% by weight aluminum.

11. In a process as claimed in claim 3, said oxygenavid alloying element consisting of at least about 0.001% by weight vanadium and said bath having less than 0.002% by weight aluminum.

12. In a process as claimed in claim 3, said oxygen-avid alloying element consisting of at least about 0.075% by weight vanadium and said bath having less than 0.002% by weight aluminum.

13. In a process as claimed in claim 3, said oxygenavid alloying element consisting of from about 0.1% by weight to about the respective eutectic composition of an element selected from the group consisting of manganese, titanium and vanadium and said bath having less than about 0.002% by weight aluminum.

14. In a process as claimed in claim 13, contacting said molten coating with air and concurrently cooling said 1 1 1 2 coating at a controlled rate for obtaining a desired oxide 3,056,694 10/ 1962 Mehler et al. 117-114 lm colour. 3,125,471 3/1964 Conner 14S- 6.35 X

15. A coloured coating produced by the process claimed in claim 13. OTHER REFERENCES 5 Fishlock: Metal Colouring, Robert Draper Ltd., 1962, References Cited Teddington, p. 280.

UNITED STATES PATENTS RALPH s. KENDALL, Primary Examiner 1,978,265 10/1934 Ivins et al. 14S-6.3 X 2,263,366 11/1941 Peck et a1. 14s-6.3 X U-S- Cl- X-R- 2,266,117 12/1941 Crocker et a1 14s-6.3 X 10 14s-6.31; 117-131 2,703,766 3/1955 Ellis et al. 117-114 

